1. Field of the Invention
The present invention relates to methods for planarizing unevenness of the seabed.
2. Description of the Background Art
The seabed has geographical features called “unevenness” which is an uneven surface. Referring to FIG. 16A, unevenness 1000 as used herein refers to a portion that is recessed with respect to the surrounding seabed 200. The seabed 200 is not flat at a location where the unevenness 1000 is present. Thus, it may not be possible to provide a structure at such a location on the seabed 200. A method that deals with the unevenness is disclosed in, e.g., Japanese Patent Publication No. 2006-322400 of unexamined applications. In Japanese Patent Publication No. 2006-322400 of unexamined applications, for installation of a foundation of an offshore wind power generation using a caisson, covering material units are positioned between a friction increasing mat provided on the lower surface of the foundation, and the surface of the seabed in order to deal with the unevenness and to increase frictional resistance. The number of covering material unit is determined corresponding to distribution of the ground level of the seabed.
According to Japanese Patent Publication No. 2006-322400 of unexamined applications, the covering material units are positioned under the foundation in the case of a structure in which the foundation has a planar bottom surface like a caisson. That is, the covering material units are positioned under the structure.
However, the structure of Japanese Patent Publication No. 2006-322400 of unexamined applications cannot be applied to, e.g., linear structures such as submarine cables. If the covering material units are positioned only under the submarine cable, the submarine cable can move onto the unevenness where no covering units are positioned, due to the tidal currents and the like. In such a case, referring to FIG. 16B, a submarine cable 20 is bent over the unevenness 1000, imposing a considerable strain on the submarine cable 20 at both ends 21 and 22 of the unevenness 1000. Thus, the submarine cable 20 can be damaged. One conventional possible solution to this problem is to repair the unevenness by dumping crushed stones with a tremie pipe or the like. However, the dumped crushed stones can be carried away by the influence of the tidal currents, whereby the unevenness can be restored. Another conventional possible solution is to repair the unevenness by installing a wire cylinder, a large concrete block, or the like therein. However, the wire cylinder, the large concrete block, or the like do not completely fit in the unevenness, and the part of the wire cylinder, the large concrete block, or the like, which does not fit in the unevenness, serves as resistance to the tidal currents, causing an excess flow. Such an excess flow causes a phenomenon called “scouring,” a phenomenon that the seabed is worn away and chipped off, near the part of the wire cylinder, the large concrete block, or the like serving as resistance. Thus, the unevenness can be restored.